1. Field of the Invention
The present invention relates to a combustion state detecting apparatus for an internal combustion engine for detecting combustion states within a cylinder of the engine on the basis of detection of an ion current.
2. Description of Related Art
For having better understanding of the concept underlying the present invention, background techniques thereof will first be reviewed in some detail. FIG. 7 is a circuit diagram showing schematically a structure of a conventional combustion state detecting apparatus for an internal combustion engine (hereinafter also referred to simply as the engine). Referring to the figure, reference numeral 1 denotes an ignition device which is implemented in the form of a transformer 11 including a primary winding 111 having a high-potential end applied with a potential of plus polarity and a low-potential end connected to a collector of a power transistor serving as a switching element 12, which transistor having an emitter electrode connected to the ground potential. The transformer (ignition coil) 11 further includes a secondary winding 112 which is coupled electromagnetically to the primary winding 111 and which has a high-potential end connected to a spark plug 3 and a low-potential end connected to a bias circuit disposed externally by way of a wiring conductor.
Further referring to FIG. 7, numeral 2 (single-dotted broken line block) denotes generally a combustion state detecting module which is comprised of a bias circuit 13 for applying to the low-potential end of the secondary winding 112 a bias voltage of plus polarity required for detecting ions produced upon occurrence of a spark discharge at the spark plug 3. To this end, the bias circuit 13 is constituted by a series circuit of a parallel connection of a diode 133 and a resistor 131 and a parallel connection of a Zener diode 134 and a capacitor 132. Connected to the output terminal of the bias circuit 13 is an ion current detecting means for detecting a current flowing through the spark plug 3 under the effect of the bias voltage charged in the capacitor 132. Further connected between the output of the bias circuit 13 and the ground is a diode 21 which forms a discharge current path for the spark plug 3.
With the structure of the conventional combustion state detecting apparatus for an internal combustion engine including the ignition device 1 and the combustion state detecting module 2 as described above, there may arise such problems as elucidated below. It is assumed that a breakage fault takes place at a circuit point (A) where the high-potential end of the secondary winding and the spark plug are interconnected or assumed that a misfire event (i.e., noncombustion of the air/fuel mixture) occurs within the cylinder in which the spark plug 3 is disposed. In that case, the voltage (having a peak level of about 40 kV) appearing at the high-potential end of the secondary winding 112 undergoes vibration, as a result of which a voltage vibration having a peak level of about 8 kV makes appearance at the low-potential end of the secondary winding 112 in synchronism with the voltage vibration at the high-potential end, although the amplitudes of both the voltage vibrations differ from each other. As a consequence, the voltage of relative high level making appearance at the low-potential end of the secondary winding 112 brings about a leak current which flows through the electronic circuitry constituting the ion current detecting means incorporated in the combustion state detecting module 2, whereby the electronic circuitry constituting the ion current detecting means may undergo damage or injury, giving rise to a problem.
As the measures for coping with the problem mentioned above, it is conceivable to accommodate the high-voltage circuitry inclusive of the bias circuit 13 within an outer packaging case which houses therein the ignition device 1 and fills the interior of the outer packaging case 16 with a curable casting resin to thereby insulate the ignition device and the high-voltage circuitry from each other. In this conjunction, it is however noted that the capacitor 132 constituting a part of the bias circuit 13 is relatively low in respect to the heat withstanding capability and thus the capacitor may provide a cause for lowering the reliability of operation of the combustion state detecting apparatus under the heat transferred from the primary winding 111, giving rise to a problem. Besides, the number of parts to be housed within the outer packaging case of the ignition device 1 will increase, which involves a problem that the size of the combustion state detecting apparatus increases unfavorably.
In the light of the state of the art described above, it is an object of the present invention to provide a combustion state detecting apparatus for an internal combustion engine which can ensure adequate thermal insulation between a transformer constituting an ignition coil and the bias circuit and which can be manufactured in a miniaturized structure.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a combustion state detecting apparatus for an internal combustion engine, which apparatus includes a transformer having a primary winding and a secondary winding which are so electromagnetically coupled mutually that upon interruption of a current flowing through the primary winding, a high voltage is induced at a high-potential end of the secondary winding, a switching means for controllably interrupting the current flowing through the primary winding, a spark plug for generating a spark discharge in response to application of the high voltage making appearance at the high-potential end of the secondary winding, a bias means for applying to a low-potential end of the secondary winding of the transformer a voltage of plus polarity for detecting ions produced when the spark discharge occurs at the spark plug, and an outer packaging case for accommodating therein the transformer, the switching means and the bias means, wherein the bias means is accommodated within the outer packaging case such that the bias means is disposed at a location adjacent to the switching means remotely from the transformer, and wherein the outer packaging case is filled with a curable casting resin.
By virtue of the structure mentioned above, the bias means can positively be protected from influence of heat generated in the primary winding, and thus there can be realized the combustion state detecting apparatus which operates with high reliability.
In a preferred mode for carrying out the invention, the combustion state detecting apparatus may further include a first wiring conductor for electrically interconnecting a low-potential end of the primary winding of the transformer and the switching means, a second wiring conductor for electrically interconnecting a high-potential end of the secondary winding of the transformer and the bias means, wherein the first and second wiring conductors are formed discretely from each other, and wherein the second wiring conductor is disposed at inner side relative to the first wiring conductor.
Owing to the structure mentioned above, degree of freedom in disposing the second wiring conductor relative to the first wiring conductor increases, whereby an adequate insulation distance can easily be ensured between the bias means and the transformer notwithstanding of a narrow internal space available within the outer packaging case.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.